A typical color characteristic measurement apparatus and method of an image pickup machine in a related art are shown in Section 3, Clause 18 of “International Standard IEC 1146-1 Video cameras (PAL/SECAM/NTSC)—Method of Measurement—Part 1: Non-broadcasting single sensor cameras” published in May 1994 by IEC (International Electrotechnical Commission) as international standard.
FIG. 48 is a drawing to show the configuration of an apparatus for measuring color and gradation characteristics of a digital still camera which is used as an example of an image pickup machine, as application of such a measurement apparatus and method in the related art. In the figure, numeral 6 denotes an image pickup machine such as a digital still camera whose color characteristic is to be measured, numeral 1 denotes a test chart of a subject of the image pickup machine 6, numeral 5 denotes an illumination light source having stable illumination intensity and color temperature for illuminating the test chart 1, and numeral 19 denotes a computer for receiving data output from the image pickup machine 6.
FIG. 49 is a format drawing of the test chart 1 containing white, black, a gray scale 200 changing gradually from white to black, and several color charts 210 of red, green, blue, etc., as reference colors. As examples of these color charts, the characteristics are defined in Annex A and Annex B of the above-mentioned international standard.
First, the R, G, and B values of the color charts 210 of the test chart 1 shown in FIG. 49 are assumed to be already known and are adopted as theoretical values. For example, if data consists of eight bits, ideally R=255 and G=B=0 for red, G=255 and R=B=0 for green, and B=255 and R=B=0 for blue.
Next, the differences between the R, G, and B values corresponding to the color charts 210 measured when an image of the test chart 1 is picked up by the image pickup machine 6 and the theoretical R, G, and B values (color differences) are found, whereby the color reproducibility of the image pickup machine 6 can be evaluated.
The gradation characteristic of the image pickup machine 6 can be found from the measurement values provided when the gray scale 200 changing gradually from white to black is imaged by the image pickup machine 6.
However, the illumination intensity and chromaticity on the face of the test chart illuminated by the illumination light source vary from one position to another. Thus, if a photograph of the same color chart is taken, the measurement value varies depending on the position on the test chart and unless illumination intensity and chromaticity inconsistencies are corrected, precise values cannot be provided.
If a photograph of the same color chart is taken even under ideal uniform illumination, the measurement value varies depending on the characteristics of the image pickup optical system of the image pickup machine, for example, because of the light quantity difference between the center and peripheral portions. Thus, unless the characteristics of the image pickup optical system of the image pickup machine are already known and are used to correct the measurement values, precise values cannot be provided.
To take a photograph of the test chart at a place where the characteristics of the image pickup optical system of the image pickup machine are the same, the test chart must be moved or replaced or the image pickup machine must be moved.
Some image pickup machines have automatic correction functions of an automatic exposure function, an automatic gain correction function, an automatic white balance correction function, etc. If a subject changes, a correction is made according to the setting responsive to the subject, thus a photograph of a different subject cannot be taken under the same setup conditions.
Although the color differences between the measurement values of the R, G, and B values corresponding to the color charts on the test chart and the theoretical R, G, and B values can be found, the spectral sensitivity characteristic of the image pickup machine as the basis of color management cannot be measured.
The test chart used as a subject usually is printed matter and it is difficult to generate black with reflectivity close to 0% and white with reflectivity close to 100%, thus the maximum value and minimum value of measurement data cannot be corrected.
Since the test chart is printed matter, change with time, such as color deterioration or color change, is involved, thus measurement with high reproducibility is hard to execute.
Only with limited types of images picked up by the image pickup machine, the color difference from any other general subject is not seen.
Only with information provided from limited types of images picked up by the image pickup machine, a highly accurate color correction cannot be made to the image of any other general subject.
To relate measurement data of color characteristic measurement apparatus to the images picked up by the image pickup machine, work such as preparation of an additional correspondence table becomes necessary.
If the type of illumination light source (spectral distribution characteristic) changes, the data corresponding to each color chart imaged by the image pickup machine also changes. However, means for accurately reflecting the characteristic of each illumination light source is not provided in related art, thus color management of the image pickup machine containing the illumination light source is hard to accomplish. Particularly, there is not a method of correctly measuring the spectral sensitivity characteristic of the image pickup machine for the reason as described above, thus if the spectral sensitivity characteristic of a light source, etc., is measured accurately, it does not lead to color management making the effective use of the spectral sensitivity characteristic.